On October 27th, 2017 comet 96P/Machholz reached its 6th Perihelion passage since it’s discovery in 1986. At the time Don Machholz discovered this comet it had already passed perihelion. 96P/Machholz is a rather unusual comet, in the case of its composition (raltively low in Cyanogen) and unusual orbit for a short periodic comet (very inclinated and eccentric).
In this (unusually long) post I will go into some depth regarding the inbound journey and perihelion passage of 96P/Machholz as seen by the Solar and Heliospheric Observatory (SOHO), Solar TErrestrial RElations Observatory Ahead (STEREO-A) and by some ground-based observations.
Before perihelion, the comet was only visible from the Southern-Hempisphere, quickly moving North and rapidly brightening. The comet wasn’t observable for very long, as it stayed extremely faint until about a month before perihelion. Soon, by mid-October, it entered daylight as seen from Earth. On October 2nd and 8th Michael Mattiazzo (Australia) estimated the comet to be around +15 mag and +13 mag respectively (see below). Rob Kauffman (Australia) estimated it to be around +13 mag on October 10th.
Comet 96P/Machholz imaged on October 2nd (left) and October 8th (right). At this stage the comet was still very faint, but rapidly brightening. In the first image the it was close to mag +15, while on Oct 8th the comet had already brightned to +13 mag. (c) Michael Mattiazzo.
Days later the comet was already close to +11 mag, and was hence detectable in SOHO’s SWAN images. In these images it was possible to follow up on the brightening, as the object had passed within the Daylight zone as seen from Earth. By late October the comet passed behind the “Sun-Occulted zone”, meant to block out the intense Ultraviolet sunlight from saturating the SWAN images.
SOHO/SWAN image extracts showing 96P/Machholz brightening as it was approaching the Sun (behind the blacked-out zone). Image credit: ESA/NASA/LATMOS SOHO/SWAN
STEREO HI1-A Observations
At about the same time the comet was passing behind the occulter in SWAN images, on October 23rd, it entered the FOV of the STEREO/SECCHI HI1-A imager. In these images the comet was bright. It was also showing a long (several degrees, and growing) prominent tail. Consequently, by the time the comet had left the FOV on October 24th, the tail was still visible after many hours. Furthermore, one could clearly see the impact of the Solar wind on the comet’s tail, causing it to appear discontinous and curvy.
Comet 96P/Machholz entering STEREO’s HI1-A FOV (October 23rd) and the comet on October 25th when it had already left the FOV. Notice how the tail still persists almost a day after the comet left the FOV! Furthermore, notice the tail’s discontinous and curvy nature, caused by its interaction with the Solar Wind. Image credit: NASA STEREO/SECCHI HI1-A.
On october 25th, only a couple of days before perihelion, the comet entered the SOHO LASCO C3 images. Based on these images the comet appeared close to mag +2. It kept at about this brightness during its passage in SOHO/LASCO. The comet left the SOHO/LASCO C3 FOV on October 29th. In these images it was possible to distinguish two tails.
Composite image extract showing comet 96P/Machholz’s at different times when transiting the SOHO/LASCO C3 FOV. Image credit: ESA/NASA SOHO/LASCO C3
Between October 27th and 29th, amateurs reported three faint fragments leading the comet in SOHO/LASCO C3 images. Two of them correspond to 96P/Machholz-B and 96P/Machholz-C, discovered during the comet’s passage in SOHO/LASCO in 2012. The third fragment is a new discovery. The comet’s were solely observed by SOHO, not even STEREO/SECCHI images could detect them.
SOHO/LASCO C3 (long-exposure) extract showing 96P/machholz and its three faint leading fragments. Two of them correspond to 96P/Machholz-B and C, discovered in 2012. The third one is a previously unknown fragment. Image credit: ESA/NASA/NRL Karl Battams.
Fragmentation events are nothing new to this comet. In fact, it’s likely that the precursors of the Marsden and Kracht-I comet groups were chunks that broke off from 96P/Machholz somewhere around 1000 years ago.
STEREO COR-A Observations
On October 26th comet Machholz entered the STEREO/SECCHI COR2 FOV. In these images the comet was a prominent object. At its best it appeared to be around mag +2 and showed a long and prominent tail. The comet left the FOV on October 28th.
STEREO COR2-A image extracts showing Comet 96P/Machholz before perihelion and after. Notice the prominent nature of this object in these images. Image credit: NASA STEREO/SECCHI COR2-A.
The comet also rapidly transited the COR1-A FOV on October 27th, near perihelion. Unfortunately the comet was hardly detectable in these images, as the it was practically on the other side of the Sun as seen from STEREO-A.
STEREO COR1-A image extract showing 96P/Machholz. In these images the comet was hardly detectable, this image is perhaps one of the better COR1-A images of it. Image credit: NASA STEREO/SECCHI COR1-A.
Accordinag to Alan Hale (USA), the comet is expected to stay unobservable from the ground until about February 2018, when the comet will start entering the night sky again. Until then, The SOHO/SWAN images will have a good view of the comet, when it exits the Sun-occulted zone of those images (beginning of November).